Ahmed Najib Chebbi, the spokesman for the Salvation Front, said on Wednesday that they would not take part in a process that lacked legitimacy and was based on exclusion.
He also accused the electoral commission of falsifying the results of a recent constitutional referendum.
The Salvation Front brings together various groups opposed to Mr Saied, including the Islamist Ennahda Party. The front also boycotted July’s referendum, but sitting out of December’s elections would be the first time they have not contested the legislature since the 2011 revolution.
“Participation in these upcoming elections will only be decorative, similar to what used to happen during the time of former president Zine El Abidine Ben Ali,” Mr Chebbi said.
The Free Dostourian Party (PDL), another major political force in the country, also announced that they would sit the elections out.
Party president Abir Moussi, a vocal critic of Ennahda, called the elections “undemocratic” and said she would lead a national struggle against the president’s policies.
Opposition groups decried July’s referendum, in which only 30.5 per cent of the electorate took part, to approve a constitution drafted by the president and a small team of experts.
While Mr Saied insisted the revisions “return Tunisia to the revolutionary path” and tackles corruption, critics have said they would concentrate power in the hands of the president at the expense of Parliament and the judiciary.
Meanwhile, the deadline for the president to announce the new electoral law for December’s vote approaches but no document has yet been released.
At a meeting with the head of the electoral commission Farouk Bouasker on Monday, Mr Saied said groups that boycotted the July referendum would be excluded from consultations over the draft.